1. Field of the Invention
Embodiments of this invention relate generally to glide testing for detecting asperities on magnetic media of the type generally used for storing digital data, and in particular embodiments to methods for accurately measuring the height of asperities independent of where the asperity contacts the glide head assembly, and systems incorporating the same.
2. Description of Related Art
Modern computers require media in which digital data can be quickly stored and retrieved. Magnetizable (hard) layers on disks have proven to be a reliable media for fast and accurate data storage and retrieval. Disk drives that read data from and write data to hard disks have thus become popular components of computer systems. To access memory locations on a hard disk, a read/write head is positioned slightly above the surface of the hard disk while the hard disk rotates beneath the read/write head at an essentially constant number of revolutions per minute (RPM). By moving the read/write head radially over the rotating hard disk, all memory locations on the hard disk can be accessed. The read/write head is typically referred to as "flying" head because it hovers above the surface on an air bearing located between the hard disk and the head, caused by rotation of the hard disk at high speeds.
Asperities, which are essentially protrusions on the surfaces of the hard disks, may cause problems when encountered by the read/write head. These asperities can cause errors in the transfer of information or even damage to the read/write head. In an effort to reduce the number of asperities, manufacturers commonly burnish the surfaces of the disk. During the burnishing process a burnishing head, rather than a magnetic read/write head, is positioned over the hard disk in a manner similar to a read/write head. Burnishing heads may be designed as either "flying" burnishing heads which pass over the surface to be burnished, or they may be designed as "contact" burnishing heads which directly engage the asperities. During the burnishing process, the burnishing head operates to smooth out these surface protrusions.
Once the initial burnishing is complete, a glide test is performed to detect, either proximately or by contact, any remaining asperities which may come into contact with the read/write head during use. Glide tests utilize glide heads which hover and detect asperities protruding from the hard disk surface, and may be programmed to reject hard disks with asperities larger than the "flying" height of the read/write head.
A typical glide head employs a slider, a component aerodynamically configured to "fly" over the hard disk at a predetermined height for a given hard disk speed, and a block of piezoelectric material attached to the slider. Piezoelectric material produces an electric potential when vibrated or otherwise physically disturbed. When the slider contacts a surface asperity, the shock to the slider temporarily disturbs the crystalline lattice of the piezoelectric material, causing a voltage to develop across its electrodes. The voltage of the piezoelectric material exhibits a frequency response with distinct vibrational modes dependent on the type of crystalline disturbance produced, and a magnitude dependent on the size of the asperity and the location on the slider that comes into contact with the asperity.
A continuing trend in the magnetic media industry is the development of hard disks with increased recording densities. As recording density increases, read/write head size and flying height must correspondingly decrease to properly read from and write to increasingly smaller areas on the hard disk. Accordingly, for manufacturers to develop production-quality hard disks, it has become necessary to utilize glide heads of decreased size and flying height with more sensitive response characteristics.
However, as the size of glide heads decrease, it is becoming difficult to precisely control their electrical response characteristics. For example, as the size of a glide head decreases, it becomes increasingly difficult to determine the location on the slider that comes into contact with the asperity. Because the frequency response of the piezoelectric material is dependent on both the size of the asperity and the point of contact on the slider, if the point of contact on the slider is unknown then the size of the asperity cannot be accurately determined from the frequency response. This uncertainty in the size of the asperity encountered may lead to the rejection of good disks or the acceptance of deficient disks.